A disk storage system, such as a magnetic hard disk drive (HDD), uses one or more disks or “platters” as a data recording medium. The HDD records data on the disk by use of a magnetic recording head which can also reproduce data from the disk.
Increased levels of storage capacity in hard disk drives are the result of many improvements in a variety of areas, including, for example, finer head positioning, smaller read/write heads, and perpendicular recording.
An important factor in increasing storage capacity is to decrease the size of the recording (and reading) head. Such heads typically comprise multiple layers of various materials produced by photolithographic manufacturing processes, in a manner similar to those processes utilized to produce integrated circuits.
In a typical photolithographic manufacturing process, light energy is transmitted through a mask that defines particular features for a layer of the structure under construction. A typical process can include dozens of masks, which are utilized in a particular sequence. A “stepper” places each mask in sequence over the structure. Aligning the multiple individual masks is a very critical operation in such processes.
Conventional mask alignment processes utilize fiducial marks produced in a first layer to align a subsequent mask. For example, such a conventional fiducial mark comprises a periodic grating structure of varying, for example, two, heights. For alignment purposes, such a fiducial mark is illuminated by a laser to produce a diffraction pattern that is used to guide a stepper to place a next mask into proper position. A typical specification for depth of such a fiducial grating mark is that the relieved “groves” should be approximately 100 nm deep. Grooves of much less than 100 nm in depth typically do not produce sufficient pattern energy to align the mask with sufficient accuracy.
Unfortunately, it is desirable to construct magnetic read/write heads with some layers, e.g., a sensor or “read” layer, of significantly less thickness. For example, a desirable thin sensor layer is approximately 20 to 40 nm. A conventional fiducial grating mark of about 100 nm thick is unsuited to use in such a thin sensor layer.
A conventional art approach utilized to overcome the limited depth available in thin layers, e.g., layers substantially thinner than a recommended fiducial grating mark depth, is to align subsequent layers to other layers that are thick enough to accommodate a recommended fiducial grating mark depth. Unfortunately, aligning to such other layers can lead to a deleterious build up of alignment tolerances. For example, thin layer B, aligned with a mark on layer A, is produced at a maximum acceptable misalignment in a particular direction with respect to layer A. Layer C, disposed adjacent to thin layer B, is also aligned to a mark on layer A, as thin layer B is unsuitable for a conventional alignment mark. Layer C is produced at a maximum acceptable misalignment in the opposite direction with respect to layer A. Consequently, under the conventional art, even though both layers B and C are within acceptable alignment specifications with respect to their alignment targets (on layer A), it is frequently the case that layers B and C exceed acceptable alignment specifications with respect to one another. Poor alignment between layers can, for example, result in poor function of the device and/or substantial yield loss.